There are simple methods to defeating the Cyberdemon. Yes it is very strong but its missiles are actually quite slow. When it launches missiles, unless you are near a wall, where the impact of the missile is still felt, you can simply dodge the missiles with simple right to left movements. This is probably the easiest in the Watch Your Step and No Way Out levels. Soon you will be confident enough to take on Cyberdemons with nothing but a Pistol!